as margaret brenner reports, the eecretary raised the possibility of a larger role for the united >> rep states. >> reporter: during a stop here in prague, secretary of state issued a stern warning to bashar al assad not to use chemical weapons. ta she said the u.s. is planning to take action if there is credible evidence of a threat. she did not say whether there that woinformation that would suggest an attack is imminent. theire reiterated, chemical heapons are a red line for the ed statstates. >> i'm not going to telegraph in any specifics what we would do hatthe event of credible evidence that the assad regime hemicalorted to using chemical weapons against their own people. >> reporter: the international community has failed to peop intervene in the conflict that as killed nearly 38,000 syrians since the uprising began. e are ds this morning," margaret brennan, prague. >>> in japan this morning at least nine people are dead after it's unclof a highway tunnel collapsed. it's unclear why those massive concrete slabs broke away from the tunnel walls on sunday. smoke an lucy craft re